A standard worm-type extruder normally used with thermoplastic synthetic resins has a housing forming a generally cylindrical throughgoing passage which receives a worm. Granules of a synthetic resin are fed into the space between the worm and the interior wall of the passage at an upstream end thereof and the worm is rotated to displace the synthetic resin downstream while plastifying it by the combined compression and working.
In order to increase the plastification action the inner walls of the housing passage normally are formed with axially extending and inwardly opening grooves, usually of decreasing radial depth in the downstream direction. Similarly the worm is formed with one or more screwthread formations. The grooves are normally of an overall length equal to three times the diameter of the worm. Furthermore when polyolefins of medium and high molecular weight are employed the upstream portion of the housing is cooled, and a so-called thermal dam is provided part of the way along the passage so that the downstream portion of the bore or passage can remain relatively hot.
Such arrangements have proven themselves extremely efficient, in particular when used in conjunction with blow-molding apparatus. Nonetheless adaptation of these units for use with other than polyolefins of high molecular weight has not been readily possible. As the consistency of the synthetic resin being plastified changes, which consistency is normally in part a function of its molecular weight, it is necessary to change the dimensions of the screw thread formation on the worm. This can most easily be done in the normal extruder simply by axially withdrawing the worm from the housing and replacing it with another worm. Although it would be desirable normally to also change the grooves inside the passage or bore when changing resin, in practice such an alteration is almost impossibly complex, even in housings whose bores or passages are at least partially defined by inserts.
Another problem with the known extruders is that materials in the resin, such as the oxides often used to color them, are extremely hard so that the worm and housing bore are subject to considerable abrasion. Thus with time it is often necessary to replace the worm and rebuild the housing with parts constituting a new bore. Replacing the worm is an easy task, but rebuilding the housing is, as mentioned above, an extremely onerous and difficult operation, particularly in view of the fact that the parts forming the bore are normally themselves formed with internal passages that permit their cooling.